1. Field of the Invention
This invention relates generally to scanning force microscopes, and more particularly to such a microscope with an independent stationary photodetector located away from a scanner, which allows the surface of samples to be imaged while the cantilever portion of the scanner is immersed in liquids without special set-up or special adapters.
2. Description of Prior Art
Scanning force microscopes (SFM), also known as atomic force microscopes (AFM), are useful for imaging objects as small as atoms. The scanning force microscope is closely related to the scanning tunneling microscope (STM) and the technique of stylus profilometry. However, in a typical scanning force microscope, deflection of a laser beam by a vertical movement of a probe following the contours of a specimen is amplified by a reflective lever arm to which the probe is mounted. The deflection of the laser beam is typically monitored by a photodetector in the optical path of the deflected laser beam, and the sample is mounted on a stage moveable in minute distances in three dimensions so that the sample can be raster scanned while the vertical positioning of the probe relative to the surface of the sample is maintained substantially constant by a feedback loop with the photodetector controlling the vertical positioning of the sample. Thus the SFM maps a spatially varying surface into an image.
The sample is mounted on piezo-ceramic transducers in such a way that the X-Y dimensions can be scanned as a raster and the Z dimension controlled through the feedback loop with the photodetector such that the tip maintains a constant force on the surface. Appropriate control electronics and computers are integral to the instrument and are used to control all movements and to acquire and display data.
The probe consists of a cantilever with a sharp tip located orthogonally on one end of the cantilever, in, or nearly in, contact with the surface to be profiled or otherwise examined.
The cantilever has such a small spring constant that typically less than one nanonewton of force will cause a noticeable deflection. The cantilever deflects due to natural forces present between the tip and the sample. The probe may be either attracted to the surface or repelled by the surface depending on the forces at work. When relative motion in the X and Y directions exists between the probe and the sample surface, the cantilever will bend as topographical features of the sample move under the tip.
Such scanning force microscopes are useful for imaging a sample which is moved in three dimensions by the scanning assembly moving the sample. However, this conventional design is only useful for samples which are comparatively small or which can be cut small enough from a larger specimen to be examined on the scanning stage of the microscope. Such samples typically must also weigh about a gram or less, in order to accommodate the relatively high scan rates without causing distortion due to resonance effects. Although previous designs for a free standing scanning force microscope include a scanning mechanism and sensor element for scanning large samples, they are difficult to operate and employ a sensor head force on a sample which prohibits use of the microscope for scanning many polymers and biological molecules without the use of special adapters or set-up, such as the use of sealed liquid cells.
It would, therefore, be desirable to provide a scanning force microscope with the capability of scanning a sample in contact with a fluid, without the preparation of special fluid cells or special adapters. This capability would be particularly useful in some applications, as such a fluid environment can significantly change scanning conditions and opportunities, and can improve the quality of the image of the sample developed by the instrument.
In conventional scanning force microscopes, a probe can be damaged by too abrupt an approach to a sample before feedback position control is actually engaged. The user typically can not easily view the approach of the lever arm and probe assembly to the surface of the sample to insure precise positioning of the probe. Even where an optical microscope is used in conjunction with the scanning force microscope to view the sample, the arrangement of the optical microscope with the scanning force microscope can be inconvenient and clumsy, and can interfere with the operation of the scanning force microscope. It would further be desirable to position X, Y, and Z scanning elements (e.g., piezo drives) in locations protected from abuse. The present invention meets these needs.
Existing SFM designs require that the operating characteristics of the tube pieces remain precisely the same. Present designs further suffer from the handicap that the cantilever is placed in precisely the same position at the end of the tube. Other existing designs are limited by the fact that the scanning head is entirely integral, not permitting scanners with different scan ranges to be used.
One common instrument design uses an optical lever arm to amplify the very small vertical motions as the contacting probe follows the contours of a surface. A sharp tip, located at the bottom end of a very small spring-board cantilever, is brought into contact with the surface to be scanned. The defection of the cantilever is monitored by a laser diode light beam which focuses onto the upper side of the end of the cantilever and reflects back onto a photodetector. Existing instruments using the optical lever arm design have several shortcomings from the user's point of view. Design simplicity, machine robustness and user friendliness are often short changed in existing designs.
Another SFM has been proposed, based upon the work of Jung and Yaniv (Elect. Ltrs. 29-3, 264-266 (1993)). This SFM obtains stationary sample stages with an optical-lever means, but uses different scanning means with correspondingly different piezo transducers. The scanning range is not limited by the size of the mirror deflecting the scanning beam, but can be extended further than previous designs until limited by the sensitivity of the piezo transducers.
Other designs require the sample to be physically attached to the scanner. Changing the scanner therefore requires the removal of the sample and reassembly of the SFM--a great inconvenience.
Another SFM has been introduced with a scanning mechanism and sensor element integrated into a package for scanning large samples. The SFM is based on the work of Hansma, et.al., (J. Appl. Phys. 76, 796-799 (1994)) and incorporates a means for tracking the beam from a stationary laser onto a cantilever located at the end of a piezo tube scanner. Tracking of the beam on the cantilever is accomplished by placing a lens within the piezo tube approximately mid-way such that as the tube scans, the laser beam remains focused on the cantilever. The deflected beam is collimated by a lens onto a stationary split detector for sensing the small deflections of the cantilever.
Such an arrangement operates in a rudimentary fashion, provided that the operating characteristics of the piezo tube remain precisely the same and provided that the cantilever is placed in precisely the same position at the end of the tube. Both of these constraints pose potential problems. The Hansma approach does solve the problem of relative motion between the focused spot on the specimen and cantilever. However, this design is limited in several ways. Some of these problems are alluded to in Hansma's paper (p.796). For example, alignment of the laser beam onto the cantilever is difficult, requiring the user of the machine to "pre-align" the cantilever onto a holder which is then carefully placed at the end of the tube scanner. Also, the piezo tube length must be relatively long (at least five centimeters) in order to obtain a large scan range of approximately 100.times.100 microns. This greater size in turn aggravates errors resulting from thermal drift effects. Such a long tube compromises the scanner's response to high frequencies, and lowers the resonant frequency, resulting in a limited scanning range. The piezo transducer scanner movement in the Z, or vertical, dimension is limited to about five microns only with this design. Such limited vertical movement means that the device's utility in practice is marginal. In addition, such a piezo tube requires high voltage or amps, which increase the cost and complexity of the electronics interface.
The Hansma approach also suffers from the fact that the scanning head is not entirely integral, not permitting scanners with different scan ranges to be used.
U.S. Pat. No. 5,157,251 refers to an SFM having a movable specimen holder housed in the base of the SFM and positioned relative to a probe housed in a sensor head. The specimen is monitored by an optical deflection detection system. This design, though, suffers from the drawback that only small specimens can be examined. Because the scanner is fixed while the specimen is moved, only specimens small enough to fit within the specimen holder can be scanned. Additionally, no liquid cells or specimens in liquid may be scanned because of the inability to seal the specimen in a fluid or gaseous environment.
U.S. Pat. No. 5,291,775 relates to an SFM which includes integrated optics for viewing the optical lever arm, probe and sample and incorporates an improved mount for the probe, which is magnetically secured to the stationary body of the SFM to improve the ease of handling. With this SFM the cantilever is stationary while the specimen is scanned. This design, however, does not allow the scanning of samples immersed in liquids without the use of special set-ups or adapters. Moreover, because the detector is an integral part of the scanning assembly, it requires that the cantilever be placed in exactly the same position at the end of the tube. Consequently, this design only allows the scanning of a relatively small specimen. In addition, the specimen is not easily viewed by the unaided eye when the SFM is in scanning position. Moreover, viewing of the specimen with an optical microscope is only permissible at a maximum of 45 degrees.
U.S. Pat. No. 5,319,960 describes a sensor module for SFM designs which uses integrated scanning drivers to allow the examination of varying sizes and weights of specimen. This module, however, does not address larger design drawbacks such as requiring the use of a fixed scanner. Similarly, the module does not ameliorate the difficulties associated with having the cantilever positioned in one fixed location at the end of the tube. Also, because the structure containing the laser, cantilever and detector is relatively large, the scan rate of the specimen in the X, or horizontal, and Y, or lateral, dimensions is severely limited owing to corresponding resonance effects. In addition, the design is somewhat delicate in operation. Inadvertent misuse can result in damage to the Z or vertical, piezo driver. As before, direct viewing of the specimen is not possible while the microscope is in the scanning position. Similarly, viewing of the specimen with an optical microscope is only possible up to a 45 degree angle. In addition, because the mechanical loop between the cantilever tip and the specimen surface is relatively great, errors resulting from thermal drift effects in the X, Y and Z dimensions are more likely.
U.S. Pat. No. 5,388,542 is directed to an SFM with a piezo-ceramic tube with means to allow the probe at its free end to move in the X, Y and Z directions. This design is substantially similar to the Hansma design and therefore does not address the disadvantages which afflict that design as discussed above. While the design may be conceptually appealing, in practice it may be difficult to maintain beam alignment with varying types and sizes of cantilevers. Typically, different cantilevers must be employed for different scanning modes and different types of microscopy.
U.S. Pat. No. 5,406,833 relates to an SFM with a "spring element" with a detecting tip whose displacement is determined by irradiating a laser beam onto the rear surface of the spring element and detecting the displacement using a multi-segmented photodetector. This complex design, however, suffers from multiple resonant frequencies resulting from the module's relatively great size. In addition, no top viewing is possible at a high angle, although viewing at approximately 45 degrees may be possible.
U.S. Pat. No. 5,408,094 similarly relates to an SFM which instead uses a cantilever with a detecting tip on the free end and a reflective surface on the rear of the tip. This design similarly is affected by multiple resonance frequencies caused by its relatively large structure. Greater disadvantage, though is that this SFM can operate with limited scan ranges. This occurs because of the difficulty in maintaining the light beam on the detector. Also, the design requires that the objective lens for viewing the sample is integral to the scanner, thus not allowing for change in magnification or for the use of specialized multiple lens optics that are typically incorporated into high quality microscopes. The integral "half-mirror" in the design further limits the quality of the viewing image.